This invention relates to semiconductor devices including unipolar barrier-forming means and having an improved voltage blocking characteristic, particularly but not exclusively Schottky diodes designed for blocking voltages of at least 50 volts.
Schottky diodes with high breakdown voltages are described in the article of that title by B. M. Wilamowski in Solid State Electronics, Vol. 26, (1983), No. 5, pages 491 to 493. The disclosed device comprises a semiconductor body a portion of which is of one conducting type, unipolar barrier-forming means which form an active barrier with said body portion at a plurality of separate areas, and closely-spaced field-relief regions which protrude to a depth in the body below the active barrier. The separate areas of the active barrier are located between the closely-spaced field-relief regions. The device has an improved voltage blocking characteristic due to the field-relief regions being sufficiently closely spaced that depletion layers extending in the body portion from neighboring field-relief regions merge together under reverse-bias of the active barrier. The unipolar barrier-forming means is a metal-based layer which forms a Schottky barrier with the body portion.
In this improved Schottky diode disclosed by B. M. Wilamowski the field-relief regions are in the form of a grid of regions in the body which are of opposite conductivity type and which form a p-n junction with the body portion. These regions, which are also contacted by the metal-based layer, act as a screen to lower the electrical field near the barrier at the surface during reverse bias of the barrier. As a result, the blocking capability is improved by a reduction of the reverse leakage current, a sharpening of the breakdown characteristic and an increase of the breakdown voltage, e.g. from 43 volts to 135 volts. When the Schottky barrier is forward biased the field-relief regions are considered to be inactive since the forward voltage drop on the Schottky barrier is smaller than the forward voltage drop would be on the p-n junction for the same current density.
It is necessary to enlarge the device area in order to accommodate both the field-relief regions and a given area for the Schottky barrier. This increases the junction parasitic capacitance, and so can reduce the switching speed of the device particularly at low voltages. However, experiments on minority carrier storage effects conducted by the present inventors indicate that a significant reduction in the switching speed of this previously-disclosed device can result from minority carrier injection into the body portion at the p-n junction when the device is forward biased. The present invention is based on the recognition that when operating at high current densities or with unipolar barriers having a high barrier this p-n junction must act as an efficient injector of minority carriers which will seriously degrade the frequency response to the overall structure.